As the popularity of the Internet has grown, and with it a vast increase in content available from the Internet, many users have found that access via the Public Switched Telephone Network (PSTN), with a maximum transfer rate of approximately 56 kbps, is unacceptably slow. Various higher-speed Internet access arrangements have been offered as an alternative to PSTN access, but one—broadband access provided via transmission plant operated by CATV carriers—has so far achieved significantly greater market penetration than the others. In a typical application for provisioning broadband data services over a cable network, one or more channels of the cable systems are allocated for downstream traffic to the data-access subscribers, while upstream traffic may be carried either in a channel established at a frequency other than those used by downstream transmissions, typically a channel established at a frequency below the frequency band used for television channels, or via a separate PSTN connection. A cable modem termination system (CMTS) provided at the cable system headend communicates through the data channel (or channels) with multiple cable modems located at subscriber premises on a shared-bandwidth basis.
In order to facilitate interoperability among cable modems manufactured by different vendors, a cable modem standard was adopted in 1997, called the Data Over Cable Service Interface Specification (DOCSIS). Among other things, the DOCSIS standard specifies modulation schemes and protocols for the exchange of bidirectional signals over cable. For the downstream transmission of data under the DOCSIS standard, the data information is modulated onto an RF carrier with a quadrature amplitude modulation (QAM) format. Specifically, the DOCSIS downstream data modulation is either 64 QAM or 256 QAM—the larger constellation 256 QAM providing a faster data rate, but also requiring a higher signal-to-noise ratio.
While the primary application of DOCSIS-compliant cable modems is the modulation and demodulation of broadband data transmitted via a cable system, the technology is equally applicable to data transmission via other transmission media having the requisite bandwidth. Among such other transmission systems are fixed wireless broadband systems, such as a Microwave Multi-point Distribution System (MMDS). However, such wireless broadband systems, as well as some cable systems could have reception issues that would not allow cable modems to operate properly when using the DOCSIS standard 256 QAM or 64 QAM modulation for the downstream channel. For example, wireless systems may be subject to variations in noise levels associated with a wireless environment. Also, certain cable systems may be subject to undesirably high noise conditions—e.g., systems that have not upgraded the trunk of the distribution system from coax to fiber, and systems where, in an effort to utilize all available bandwidth of existing equipment, channels have been added near the limits of the available bandwidth. Noise conditions in such wireless, wired, optical, coaxial, cable, and Hybrid Fiber-Coax (HFC) systems could cause a typical bi-directional communication device, such as a modem, to operate improperly or unreliably.